The role of mouse mammary tumor virus (MTV) in the transformation of mammary gland cells was investigated both in vivo and in vitro. The mammary epithelial cells from preneoplastic mammary glands were cultured, and virus-cell interaction was investigated by light and electron microscopic techniques. Cell lines from various stages of neoplastic conditions were established. By virtue of the fact that the MTV type B as well as its precursor type A particles were recognizable only in the primary cell cultures and they disappeared in the later stages, hormone manipulation is being undertaken to reactivate the virus particles in the established cultures which may reveal the role of the MTV in the transformation of mammary gland cells. Electron microscopic studies of human breast tumors is being continued to detect virus-related entities, e.g., "small particles" which will be isolated for biochemical analysis. BIBLIOGRAPHIC REFERENCE: Knazek, R., Lippman, M. E. and Chopra, H. C.: Formation of solid human mammary carcinoma in vitro. J. Natl. Cancer Inst. 58: 419-422, 1977.